Photocathode-phosphor imaging system for x-ray camera tubes

ABSTRACT

An improved system for emission of electrons in response to xrays comprises a phosphor layer separated from a photocathode layer by a light transparent barrier layer which protects the phosphors and the binders in the phosphor layer from attack by the chemicals in the photocathode layer. The system is useful in various types of camera tubes including image orthicons and intensifier vidicons as well as direct view tubes to convert a light image of X-rays into a corresponding electron emission pattern.

United States Patent Fister et al.

[4 1 Dec. 19,1972

[54] PHOTOCATHODE-PHOSFHOR IMAGING SYSTEM FOR X-RAY CAMERA TUBES [72] Inventors: Karoly G. Fister; Martin ll). Gibbons, both of Camillus, N.Y.

[73] Assignee: General Electric Company [22] Filed: Jan. 29, 1971 [21] Appl. No.: 111,045

[52] U.S Cl. "250/833 R, 250/71.5 R, 250/80, 250/213 VT, 252/192, 313/65 [51] int. Cl......' ..G01t 1/16, HOlj 39/00 [58] Field 01 Search.....250/80, 213 VT, 71.5 R, 83.3 R; 252/192; 313/65 [56] References Cited UNITED STATES PATENTS 2,798,823 7/1957 Harper ..250/80 3,436,550 4/1969 Finkle ..250/2l3 VT OTHER PUBLICATIONS The Condensed Chemical Dictionary 6th Ed. p. 316 (411966) Reinhold Pub. Corp., NY.

Primary Examiner-David Schonberg Assistant Examiner-Robert L. Sherman Attorney-Nathan J. Cornfeld, John P. Taylor, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman [57] ABSTRACT 4 Claims, 2 Drawing Figures PATENTED DEC 1 9 I972 FORM BARRIER LAYER OVER COATED SUBSTRATE.

ASSEMBLE SUBSTRATE INTO CAMERA TUBE.

FORM PHOTO CATHODE LAYER OVER BARRIER LAYER.

INVEHTflFS; KAROLY G. FISTER, MARTIN D. GIBBONS,

THEIR ATTORNEY.

PHOTOCATI-IODE-PHOSPHOR IMAGING SYSTEM FOR X-RAY CAMERA TUBES BACKGROUND OF THE INVENTION This invention relates to imaging systems for camera tubes. More particularly the invention relates to an imaging system which emits electrons in conformity with a pattern of electromagnetic radiation imaged thereon.

While the conversion of a light pattern to an electron pattern in its simplest form is usually accomplished by imaging a light source directly on photosensitive material such as alkali metals or the like. Electro-magnetic radiation of particular wavelength; such as, for

example, X-rays, poses aproblem because suitably responsive photoemissive material is not always available or if available, not necessarily satisfactory or economical.

A satisfactory alternate approach is to choose a phosphor which is stimulated by the particular wavelengths of the radiation source to emit light radiation which, in turn, causes electron emission from photocathode materials responsive to the particular wave-lengths of the light emitted from the phosphor.

Formation of such devices has been difficult, however. Photocathodes are normally made by evaporating metals onto a support. If the metal vapor comes in direct contact with the phosphor layer, breakdown of the binders in the phosphor layer can occur with the byproducts poisoning the photocathode. The phosphors themselves are subject to attack by some of the metals used in photocathode formation.

Prior art attempts to solve these problems include: placing the phosphor layer on one side of a glass substrate and the photocathode on the other side; and placing a thin layer of oxidizable metal between the phosphor and the photocathode. The first method requires the use of a glass substrate of about mils thickness coated on one side with phosphors and with photocathode material on the other side. This results in a very fragile device while the use of thicker glass results in loss of resolution. In the other prior art construction, aluminum oxide barriers sufficiently thick to prevent diffusion are tediously constructed by laying down successive thin layers of aluminum metal and oxidizing each layer before evaporating the next layer of aluminum metal until the required thickness is attained. The result is a time-consuming and costly construction.

Furthermore, the barrier layer must be relatively diffusion proof and yet transparent to light. It also must be compatible with the photocathode materials subsequently vaporized thereon. The prior art approaches have not satisfactorily solved all these problems.

Quite surprisingly, we have discovered materials which adequately protect the phosphor layer, allow good transmission of light therethrough; are compatible with the photocathode materials subsequently vaporized thereon; and are easily evaporated onto the phosphor layer in a single evaporation step.

SUMMARY OF THE INVENTION Briefly stated the invention comprises a system for the emission of electrons in response to a pattern of light imaged thereon comprising a phosphor layer and a photocathode layer with a barrier layer therebetween comprising a thin layer of light transparent material capable of protecting the phosphor layer from attack by the materials in the photocathode layer, yet compatible with the photocathode material. In one aspect the invention relates to a device having an alkali metal aluminum fluoride as a barrier material between the phosphor layer and the photocathode layer.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a process flow sheet. FIG. 2 is a fragmentary cross-sectional view illustrating the invention.

DETAILED DESCRIPTION Referring now to the drawings the flow sheet of FIG. 1, outlines the process resulting in the structure shown in FIG. 2 wherein the layers are shown not to scale to better illustrate the relationship.

In one embodiment the device is constructed by first coating a substrate 2 with a phosphor. Substrate 2 must be light transparent to the wavelengths which will be imaged thereon. For simplicity of manufacture, it is preferred that the substrate be thick enough to withstand atmospheric pressure when the substrate is incorporated into the evacuated envelope of a camera tube as one wall thereof. For X-ray use, glass or aluminum are examples of satisfactory substrate material.

The phosphor material, which is selected to be responsive to the wavelengths of light to be imaged thereon, for example, silver-activated zinc sulfide for X-rays, is either evaporated directly onto the substrate or preferably suspended as particles of about 50 microns diameter in a silicon resin system. The suspension is coated onto the substrate such as by spraying or the like to form a layer 4 of about 10-20 mils thickness.

The coated substrate is air dried and then baked in an air-circulated oven at an initial temperature of about 50 C which is gradually increased up to about 350 C over about a 24 hour period. The coated substrate is then cooled and transferred to a vacuum chamber where it is baked at about 300 C for an additional 24 hour period.

In another embodiment, the glass or metal substrate is omitted and a self-supporting phosphor screen is constructed by forming the phosphor-silicon resin system into a self-supporting screen layer of sufficient thickness (about 20-50 mils) to provide the necessary physical strength after the resin has cured.

After formation of phosphor layer 4, a barrier layer 6 is applied thereon. Barrier layer 6 comprises an alkali metal aluminum fluoride. In the preferred embodiment, the barrier layer comprises cryolite i.e., sodium aluminum fluoride. Potassium aluminum fluoride can also be used. Barrier layer 6 is applied to a thickness of about 20010,000 angstroms. The barrier layer may be applied, for example, by deposition by placing the phosphor screen or phosphor coated substrate in a bell jar, heafing the phosphor layer to about 300 C, and vaporizing the alkali metal aluminum fluoride material onto the phosphor layer at a pressure below atmospheric. Suitable electrodes or conductive coatings can then be placed over the barrier layer for contact with the subsequently formed photocathode layer.

Coated phosphor layer 4 is then mounted into a camera tube and a photocathode layer 8 formed on barrier layer 6. When phosphor layer 4, for example, is coated on a glass or metal substrate 2 which is to form one wall of the camera tube, the substrate is sealed to the tube, the tube evacuated and then the appropriate photocathode materials are vaporized onto the barrier layer overlying the phosphor material. The photocathode will be familiar to those skilled in the art both as to its composition and method of manufacture. Generally speaking, the photocathode comprises a mixture of at least one alkali metal with one metal. Examples of alkali metals used in photocathodes include sodium, potassium, rubidium and cesium. Examples of metals used in combination with alkali metals to form photocathodes include silver, antimony, bismuth, and tellurium.

The invention will be further understood by the following examples.

EXAMPLE I A 6 inch diameter glass dome of inch thickness was coated with a silicon resin-phosphor suspension formed by suspending 200 grams of p-20 phosphors (silver: zinc-cadmium sulfide) of 50 micron average particle size into 150 grams of a 60 percent solids silicon resin and 150 grams of toluene. The suspension was sprayed onto the dome to a thickness of about 10-20 mils and air-dried. The coating was then cured in an air circulated oven according to the following procedure:

1 hour 50 C 1 hour 80 C 1 hour 100C 2 hours 150 C 8 hours 25 C 4 hours 300 C 4 hours 340 C The coated dome was then cooled to room temperature and transferred to a vacuum chamber where it was baked for 24 hours at 300 C to remove any gases formed during curing of the resin.

The dome was then cooled to room temperature and transferred to a bell jar in a vacuum system. The dome was heated to 300 C and 20 milligrams of cryolite was vaporized from an evaporation boat to form a coating of about 300 angstrom average thickness over the phosphor coating.

The center portion of the barrier layer was then masked and a 1,000 angstrom layer of aluminum was deposited around the outer portion of the surface of the barrier layer to form a band of about rt inch.

The dome was then removed from the bell jar and sealed to an image intensifier tube. The tube was then evacuated and the photocathode layer formed by evaporating antimony, potassium, sodium, and cesium onto the barrier layer.

The resulting tube was tested by exposure to an X- ray image onto the phosphor layer. The results indicated satisfactory functioning of both the phosphor and photocathode layers thereby indicating that the barrier layer had successfully prevented any deleterious chemical reaction between the phosphor and photocathode materials.

EXAMPLE II The procedure of Example I was repeated except that magnesium fluoride was substituted for sodium 5 aluminum fluoride. The resulting tube did not perform satisfactory. Subsequent examination of the imaging device indicated that the magnesium fluoride had interacted with the photocathode and thus had presumably poisoned the photocathode.

Thus the invention provides an imaging device having a phosphor layer and a photocathode layer with a barrier layer therebetween which readily passes the phosphor emitted light therethrough; protects the phosphor layer as well as the photocathode layer from chemical interaction; is non-reactive with either the phosphor or photocathode materials; and is easily constructed without lengthy and costly processing steps.

What we claim as new and desire to secure by'Letters Patent of the United States is: v

1. A device capable of emitting electrons in response to light energy of X-ray wavelength imaged thereon comprising:

a. a first phosphor layer comprising phosphors capable of emitting visible light in response to X-ray stimulation and bonded in a silicon resin binder;

b. a protective barrier layer on said phosphor layer comprising a 200-10,000 angstrom layer of visible light transparent material consisting essentially of an alkali metal aluminum fluoride;

c. a photocathode layer on said barrier layer comprising at least one alkali metal incombination with a second metal; said protective barrier layer providing protection for said phosphors and said silicon resin, binder against chemical attack by materials in said photocathode layer. 1

2. The device of claim 1 where said phosphor layer is carried on a glass substrate.

3. The process of forming a device capable of emitting electrons in response to light energy of X-ray wavelength imaged thereon comprising:

a. forming a first phosphor layer comprising phosphors capable of emitting visible light in response to X-ray stimulation and bonded in a silicon resin binder; I

b. forming a barrier layer on said phosphor layer comprising a 200-10,000 angstrom layer of visible light transparent material consisting essentially of an alkali metal aluminum fluoride capable of providing resistance to chemical attack on said binder and said phosphors by alkali metal; and

c. forming a photocathode layer on said barrier layer comprising at least one alkali metal in combination with'a second metal.

4. A device capable of emitting a pattern of electrons from an alkali metal photocathode layer in response to light energy of X-ray wavelengths imaged on an adjacent phosphor layer comprising:

a. a phosphor layer comprising phosphors capable of emitting visible light in response to X-ray stimulation and a silicon resin binder;

b. a protective, visible light transparent, layer of sodium aluminum fluoride of about 200-10,000 angstrom thickness-on said phosphor layer to protect said phosphors and said silicon resin binder from chemical attack by said alkali metal; and

l060ll 0192 c. a photocathode layer on said protective layer comprising at least one alkali metal in combination with a second metal.

l l t l060l l Ol93 

2. The device of claim 1 where said phosphor layer is carried on a glass substrate.
 3. The process of forming a device capable of emitting electrons in response to light energy of X-ray wavelength imaged thereon comprising: a. forming a first phosphor layer comprising phosphors capable of emitting visible light in response to X-ray stimulation and bonded in a silicon resin binder; b. forming a barrier layer on said phosphor layer comprising a 200- 10,000 angstrom layer of visible light transparent material consisting essentially of an alkali metal aluminum fluoride capable of providing resistance to chemical attack on said binder and said phosphors by alkali metal; and c. forming a photocathode layer on said barrier layer comprising at least one alkali metal in combination with a second metal.
 4. A device capable of emitting a pattern of electrons from an alkali metal photocathode layer in response to light energy of X-ray wavelengths imaged on an adjacent phosphor layer comprising: a. a phosphor layer comprising phosphors capable of emitting visible light in response to X-ray stimulation and a silicon resin binder; b. a protective, visible light transparent, layer of sodium aluminum fluoride of about 200- 10,000 angstrom thickness on said phosphor layer to protect said phosphors and said silicon resin binder from chemical attack by said alkali metal; and c. a photocathode layer on said protective layer comprising at least one alkali metal in combination with a second metal. 